The present invention relates generally to a composition and method for improved reactant and coolant flow sealing within joined or fluidly cooperating fluid-delivery plates used in a fuel cell assembly, and more particularly to the use of a seal with latent adhesive properties to allow for the assembly of fuel cells where significant adhesion between joined components takes place only after the component assembly process is substantially complete.
Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
In such a stack, adjacent MEAs are separated from one another by a series of reactant flow channels, typically in the form of a gas impermeable bipolar plate that—in addition to promoting the conveyance of reactants, coolant and byproducts—provides structural support for the MEA, as well as electrical current collection or conveyance. In one common form, the channels are of a generally serpentine layout that covers the majority of the opposing generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes reactant flow to or from the fuel cell, while additional channels (that are fluidly decoupled from the reactant channels) may also be used for coolant delivery. In one configuration, the bipolar plate is itself an assembly formed by securing a pair of thin metal sheets (called half-plates) that have the channels stamped or otherwise integrally formed on their surfaces. The various reactant and coolant flowpaths formed by the channels on each side typically convene at a manifold (also referred to herein as a manifold region or manifold area) defined on one or more opposing edges of the plate. Examples of all of these features—as well as a typical construction of such bipolar plate assemblies that may be used in PEM fuel cells—are shown and described in commonly owned U.S. Pat. Nos. 5,776,624 and 8,679,697 the contents of which are hereby incorporated by reference in their entirety.
It is important to avoid leakage and related fluid crosstalk within a PEM fuel cell stack. To overcome such leakage, the Assignee of the present invention has applied a relatively thick elastomeric seal (in the form of a gasket) onto discrete portions of the relatively planar surface of the bipolar plate. While useful in establishing the requisite degree of sealing, the thick nature of the sealants makes such an approach unfeasible in actual fuel cell stacks that are made up of more than one hundred bipolar plate and MEA assemblies, as volumetric concerns—especially in the confined spaces of an automobile engine compartment—become paramount. Moreover, the difficulty of ensuring a consistent, repeatable placement of the seal makes this approach cost-prohibitive.
In an alternate to using thick elastomeric seals, the Assignee of the present invention has developed integrally formed bipolar plate sealing where stampings formed in the plate surfaces in a manner generally similar to those used to form the reactant and coolant channels produce gasket-like outward-projecting metal beads to establish discrete contact points between adjacent plate surfaces. An example of using a metal bead to promote such sealing may be found in commonly owned U.S. Pat. No. 7,186,476 the contents of which are hereby incorporated by reference in their entirety. These beads (which may be formed to define a cross sectional rectangular, trapezoidal, semi-spherical or other related shape) are more compatible with high-volume production than that of the thick elastomeric sealant mentioned above. To promote even better sealing, the Assignee of the present invention is pursuing the use of thin elastomeric seals (also referred to herein as microseals) that permit relatively prompt curing of the seals with latent adhesion properties such that robust adhesive bonding is delayed until such time as the various individual fuel cells of a fuel cell stack can be aligned and compressing within the stack housing; this is disclosed in co-pending U.S. patent application Ser. No. 15/019,152, filed Feb. 9, 2016, entitled ROBUST FUEL CELL STACK SEALING MATERIALS AND METHODS USING THIN ELASTOMERIC SEALS the contents are incorporated herein by reference in their entirety. In another attempt to promote better sealing, the Assignee of the present invention is pursuing the use of microseals where one or more design parameters (such as Poisson's Ratio, aspect ratio and surface frictional or adhesive properties) associated with the microseal can be used to impart an effective stiffness that it exhibits decreased dependence on plate-to-plate misalignment relative to conventional seals; this is disclosed in co-pending U.S. patent application Ser. No. 15/019,128, filed Feb. 9, 2016, entitled ROBUST FUEL CELL STACK SEALING DESIGNS USING THIN ELASTOMERIC SEALS.
What is needed is a material that is tailored to the needs of these improved microseals to facilitate improved fuel cell stack assembly as well as post-assembly stack sealing efficacy and reliability.